CD274 or PD-L1 is a 290 amino acid type I transmembrane protein encoded by the CD274 gene on mouse chromosome 19 and human chromosome 9. CD274/PD-L1 expression is implicated in evasion of immune responses involved in chronic infection, e.g., by viruses (including, for example, HIV, HBV, HCV and HTLV, among others), by bacteria (including, for example, Helicobacter pylori, among others) and by parasites (including, for example, Schistosoma mansoni).
CD274/PD-L1 expression is also implicated in suppression of anti-tumor immune activity. Tumors express antigens that can be recognized by host T cells, but immunologic clearance of tumors is rare. Part of this failure is due to immune suppression by the tumor microenvironment. PD-L1 expression on many tumors is a component of this suppressive milieu and may act in concert with other immunosuppressive signals. PD-L1 expression has been shown in situ on a wide variety of solid tumors including breast, lung, colon, ovarian, melanoma, bladder, liver, salivary, stomach, gliomas, thyroid, thymic epithelial, head, and neck (Brown J A et al., 2003. J. Immunol. 170:1257-66; Dong H et al. 2002. Nat. Med. 8:793-800; Hamanishi J, et al. 2007. Proc. Natl. Acad. Sci. USA 104:3360-65; Strome S E et al. 2003. Cancer Res. 63:6501-5; Inman B A et al. 2007. Cancer 109:1499-505; Konishi J et al. 2004. Clin. Cancer Res. 10:5094-100; Nakanishi J et al. 2007. Cancer Immunol. Immunother. 56:1173-82; Nomi T et al. 2007. Clin. Cancer Res. 13:2151-57; Thompson R H et al. 2004. Proc. Natl. Acad. Sci. USA 101:17174-79; Wu C, Zhu Y, Jiang J, Zhao J, Zhang X G, Xu N. 2006. Acta Histochem. 108:19-24). In addition, PD-1 expression is upregulated on tumor infiltrating lymphocytes, and this may also contribute to tumor immunosuppression (Blank C et al. 2003. J. Immunol. 171:4574-81). In ovarian cancer, PD-L1 expression is inversely correlated with intraepithelial, but not stromal, infiltrating CD8 T cells, suggesting that PD-L1 inhibits the intratumor migration of CD8 T cells (Hamanishi J et al. 2007. Proc. Natl. Acad. Sci. USA 104:3360-65). Translation of PD-L1 mRNA is enhanced by loss of PTEN and the ensuing activation of Akt, a common event in tumorigenesis (Parsa A T et al. 2007. Nat. Med. 13:84-88). Most importantly, studies relating PD-L1 expression on tumors to disease outcome show that PD-L1 expression strongly correlates with unfavorable prognosis in kidney, ovarian, bladder, breast, gastric, and pancreatic cancer (Hamanishi J et al. 2007. Proc. Natl. Acad. Sci. USA 104:3360-65; Inman B A et al. 2007. Cancer 109:1499-505; Konishi J et al. 2004. Clin. Cancer Res. 10:5094-100; Nakanishi J et al. 2007. Cancer Immunol Immunother. 56:1173-82; Nomi T et al. 2007. Clin. Cancer Res. 13:2151-57; Thompson R H et al. 2004. Proc. Natl. Acad. Sci. USA 101:17174-79; Wu C, Zhu Y, Jiang J, Zhao J, Zhang X G, Xu N. 2006. Acta Histochem. 108:19-24). In addition, these studies suggest that higher levels of PD-L1 expression on tumors may facilitate advancement of tumor stage and invasion into deeper tissue structures.
The PD-1 pathway can also play a role in hematologic malignancies. PD-L1 is expressed on multiple myeloma cells but not on normal plasma cells (Liu J et al. 2007. Blood 110:296-304). PD-L1 is expressed on some primary T cell lymphomas, particularly anaplastic large cell T lymphomas (Brown J A et al., 2003. J. Immunol. 170:1257-66). PD-1 is highly expressed on the T cells of angioimmunoblastic lymphomas, and PD-L1 is expressed on the associated follicular dendritic cell network (Dorfman D M et al. 2006. Am. J. Surg. Pathol. 30:802-10). In nodular lymphocyte-predominant Hodgkin lymphoma, the T cells associated with lymphocytic and/or histiocytic (L&H) cells express PD-1. Microarray analysis using a readout of genes induced by PD-1 ligation suggests that tumor-associated T cells are responding to PD-1 signals in situ in Hodgkin lymphoma (Chemnitz J M et al. 2007. Blood 110:3226-33). PD-1 and PD-L1 are expressed on CD4 T cells in HTLV-1-mediated adult T cell leukemia and lymphoma (Shimauchi T et al. 2007. Int. J. Cancer 121: 2585-90). These tumor cells are hyporesponsive to TCR signals.
Studies in animal models demonstrate that PD-L1 on tumors inhibits T cell activation and lysis of tumor cells and in some cases leads to increased tumor-specific T cell death (Dong H et al. 2002. Nat. Med. 8:793-800; Hirano F et al. 2005. Cancer Res. 65:1089-96). Tumor-associated APCs can also utilize the PD-1:PD-L pathway to control antitumor T cell responses. PD-L1 expression on a population of tumor-associated myeloid DCs is upregulated by tumor environmental factors (Curiel T J et al. 2003. Nat. Med. 9:562-67). Plasmacytoid dendritic cells (DCs) in the tumor-draining lymph node of B16 melanoma express IDO, which strongly activates the suppressive activity of regulatory T cells. The suppressive activity of IDO-treated regulatory T cells required cell contact with IDO-expressing DCs (Sharma M D et al. 2007. J. Clin. Invest. 117:2570-82).